Photoconductive compositions and sheets coated therewith and process for preparing them



United States Patent f PHOTOCONDUCTIVE COMPOSITIONS AND SHEETS COATED THEREWITH AND PROC- ESS FOR PREPARING THEM Joseph Shulman, Hyde Park, Mass, assignor to Arthur D. Little, Inc., Cambridge, Mass, a corporation of Massachusetts No Drawing. Filed Jan. 4, 1965, Ser. No. 423,329 7 Claims. (Cl. 96-1) This is a continuation-in-part of my copending applica tion Serial No. 44,841, filed June 25, 1960, now abandoned.

This invention relates to photoconductive compositions and sheets coated therewith, and to a process for preparing such compositions and making such sheets.

The aforesaid sheets consist essentially of paper or other suitable supporting means having on one side or both sides a photoconductive coating of an electrically insulating film-forming binder or vehicle in which zinc oxide or other suitable photoconductive substance is intimately mixed. In order to provide prints by the use of these sheets (which will hereinafter be referred to as photoconductive sheets), they are first charged upon their surfaces with a uniform electrostatic charge and the charged surface is then exposed to light through a photographic transparency or other indicia-bearing device. The portions of the surface upon which the rays of light impinge become discharged while the unexposed portions of the surface remain in charged condition. A colored powder mixed with finely divided iron filings is then brushed over the surface, whereby the powder is held electrostatically on the charged area. Surplus powder, including that on the uncharged areas, is brushed off and the remaining powder is fixed in position, as by heating to flux it into the surface, in the areas carrying the charge, thereby forming a permanent print or record upon the areas on which no light has fallen. Other methods for preparing the perma nent print from the light-exposed sheet are also known.

The usual procedures for making photoconductive sheets comprise preparing a solution of the binder in a volatile organic solvent, incorporating the zinc oxide or other photoconductive substance into the binder solution, and then coating this composition onto the supporting sheet, followed by drying to volatilize the solvent. There are, however, many disadvantages in the use of volatile organic solvents, such as fire hazard, toxicity, and cost. While these disadvantages can be overcome to a considerable extent by the use of suitable ventilating means and solvent recovery equipment, such means and equipment introduce additional costs and operating problems which it is highly desirable to avoid if feasible. As a consequence, efforts have been made to replace the volatile organic solvents with water and thus to provide for dispersing'the binder and the zinc oxide in the water to form the desired coating composition.

I have developed compositions of this type as exemplified in German Patent 958,355, issued February 2, 1957, to myself and Raymond Winfield James. Such compositions are prepared by dispersing zinc oxide in water, with the aid of a wetting or emulsifying agent, mixing this dispersion with a dispersion of binder (e.g., polyvinyl acetate) and plasticizer, and then spreading the resulting mixture on the base sheet and drying it. Other sequences of mixing may be used, but additives such as dispersants, emulsifiers, protective colloids, and plasticizers have always been required with polyvinyl acetate or any of the other binders referred to, in order to provide a homogeneous mixture which could be spread by commonly used processes and apparatus and dried to form a film which had the desired flexibility as well as its photoconductive 3,248,217 Patented Apr. 26, 1966 properties. The compositions and products disclosed in the aforesaid German patent are eminently suitable for uses and conditions where volatile solvents are to be avoided and where high humidity is not an important factor in the storage and use of the finished photoconductive sheets. I find, however, that high humidity conditions, specifically humidities of RH. and higher, have a deleterious effect on sheets so prepared. This is because the character of the coating is such that, under the influence of such high humidity, the charge on the zinc oxide or other photoconductive substance is able to leak off. The sheets must be processed very quickly if legible prints are to be obtained, and even then the results are likely to be very poor or even indistinguishable. High humidity conditions of 75% RH. (relative humidity) and above are not confined to merely a few areas of the world, but are present for example throughout most of the densely populated eastern United States during the summers. The commercial importance of the humidity problem in the field of photoconductive sheets is thus obvious.

It is, therefore, an object of this invention to prepare photoconductive sheets which are suitable for use throughout the entire humidity range to provide satisfactory sharp prints. Another object is to prepare photoconductive sheets by a process which does not require the use of volatile organic solvents, butin which water is used as the solvent or dispersing medium. Still another object is to prepare photoconductive sheets having coatings of which the binder for the photoconductive substance is of such a nature that is especially effective to resist leakage of the charge under high humidity conditions. Other objects will appear from the accompanying disclosure.

It might be expected that the sensitivity to hmidity of the photoconductive sheet would be diminished by avoiding entirely the use of water as a solvent or dispersing medium for the binder and reverting to the well-known, though more costly and less convenient, organic solvent technique. On the contrary, however, I have found that I am able to use water as the solvent or dispersing medium and yet attain excellent humidity resistance by proper selection of a binder material and of suitable conditions of treatment, as described hereinafter.

The material of which the support for the photoconductive sheet is made must be conductive enough to per mit leaking out of the charge developed in that portion of the surface film upon which the light falls. Paper is the material ordinarily used as this supporting material, but many other supporting materials known to this art may be used, particularly those of a cellulosic nature such as cellulose acetate, viscose, etc., in film form or made from fibers, e.g. by paper-making or by textile-making tech- Inques.

The photoconductive substance which is most commonly used for making photoconductive sheets is zinc oxide, as it gives good results for most commercial uses and is relatively inexpensive. However, there are many other photoconductive materials, well known and used for such effects, e.g. to obtain particular tints or color effects in the final print. Such materials include various photoconductive inorganic oxides and sulfides as well as selenium and some organic materials such as methylene blue and fluorescein.

In accordance with the present invention, I provide a coating composition which consists essentially of a resinous binder, an emulsifying agent therefor, finely divided photoconductive material, a dispersant therefor, and Water. The binder is not Water-soluble, but is emulsifiable in Water. A dispersant is useful in dispersing the photoconductive material into the water emulsion of the binder. The emulsifier and the dispersant are characterized by being either volatile, or water-insoluble and water-insensitive in the final coated sheet. Inasmuch as the resinous binder, after curing following the coating step, is water-insoluble, as is also the photoconductive material, it follows that the final cured coating is completely water-insoluble and water-insensitive.

The resinous binder of this invention is polyethylene, in the form of a water-emulsifiable resin. Water-emulsifiable polyethylene useful in the present invention are characterized by containing carboxyl groups which react with amines or other basic surfactants to form an emulsion. More specifically, they are characterized by having an acid number of at least 10. The acid number may be as high as 30, or even more, as will be evident from the range of properties described in more detail below.

A number of polyethylenes are not water-emulsifiable, e.g. the Epolene N types and the Epolene C types, both manufactured by Eastman Chemical Products, Inc., Kingsport, Tennessee. These have acid numbers much less than 10, ranging down in fact to less than 1. The Epolene E types, made by the same company, are, however, water-emulsifiable and will be referred to in more detail hereinafter.

The water-emulsifiable polyethylenes suitable for use herein may be either the low density type (LDPE) or the high density type (HDPE) as long as they possess the required properties as here set forth. The low density type must have a softening point below 115 C., and a molecular weight between about 1200 and 3500. The high density type must have a softening point below 150 C. and a molecular weight between about 6000 and 9000.

, Both types are at least 50% crystalline, the low density Acid number 14 14 Molecular weight 1, 500 3, 400 Softening point, C 108 95 Crystallinity, pereent 50-60 50-60 Density, 25 C 0. 95 0. 92

Other suitable low density water-emulsifiable polyethylenes than the Epolene E types mentioned above are those of the Emulsifiable Grade Polyethylene AC type manufactured by Eastman Chemical Products, Inc., Kingsport, Tennessee, which meet the requirements set forth above.

Among suitable high density water-emulsifiable polyethylenes are the Grex type manufactured by W. R. Grace & Co., Polymer Chemicals Division, Clifton, New Jegsey, which have the range of characteristics set forth a ove.

Many emulsifiers for polyethylene are known; I prefer to use a two-component emulsifier consisting of a volatile organic base, preferably an amine or ammonia, and a water-insoluble fatty acid. By volatile is means that the compound volatilizes completely under the conditions of heating to cure or set the resin. Typical of the organic base are 2-amino-2-methylpropanol, 3-methoxy-propylamine, morpholine, and ammonia. Some care must be exercised when using ammonia, to avoid premature volatilization during processing, although with such care it gives good results. I

Typical of the fatty acid are stearic, palmitic and oleic acids, and in general any fatty acid having 14 to 18 carbon atoms, i.e., C H COOH wherein n is 13 to 17 inclusive. These fatty acids do not volatilize during the aforesaid heating step, and being water-insoluble and Water-insensitive they tend to contribute to the water resistance of the final coating. The emulsifiers are used in an amount of about 25 to 50% of the polyethylene by weight, and preferably about 15 to 30% of the fatty acid and the balance volatile emulsifier.

Zinc oxide is the preferred photoconductive material, and is used in an amount of between one and 5 parts to one part of the polyethylene, by weight. Lesser amounts of zinc oxide produce prints of insufficient contrast, while larger amounts tend to make the coating compositions unstable and to cause coagulation thereof upon standing. A ratio of 1:3 /2 to 1:4 polyethylene to zinc oxide is preferred.

Zinc oxide alone cannot be effectively mixed with the polyethylene emulsion, because it breaks the emulsion, causing coagulation of the polymer particles. It is therefore necessary to add a dispersant in order to prevent the breaking of the emulsion. Although various dispersants may be used for the purpose of merely putting the zinc oxide into the composition, without causing coagulation, it is necessary to use one that is water-insoluble and water-insensitive in order to provide a final cured coating which answers the rigid requirements of the present invention. Otherwise, the product sheet will not have the desired printing characteristics at 75 RH. and above. Although I have tried many dispersants for this purpose, I have found that the only satisfactory ones are the water-soluble silicone resins which, on drying and curing of the coating, form a water-insensitive, water-insoluble product. This product in fact forms a protective film around each particle of the zinc oxide. Typical of these silicone resins are methyl and ethyl silicones.

The amount of silicone resin is preferably 1 to 3% of the zinc oxide by weight, although amounts ranging from 0.5 to 5% may be used. With lesser amounts, the final sheets show less resistance to the higher humidities.

The amount of water is such as to permit effective coating by the particular coating means used. It may therefore vary over a fairly wide range.

The following examples will serve to illustrate this invention in greater detail. Parts are by weight.

Example 1 A polyethylene emulsion is prepared by melting together 30 parts polyethylene AC-629 (acid number 15, molecular weight 1900, softening point 101 C., crystallinity 52%, density (25 C.) 0.93) and 6 parts stearic acid, and then adding 3 parts 2-amino-2-methylpropanol with the melt temperature at 248266 F. The melt is then added slowly, with rapid stirring, to 105 parts water at 200-205 F. The resulting emulsion is then cooled to ambient or somewhat above and water is added to make up any evaporation losses.

A zinc oxide dispersion is prepared by mixing 300 parts zinc oxide, 300 parts Water, and 8 parts water-soluble methyl silicone resin (DC-7 72, Dow Chemical Company, Midland, Michigan). One part of the resulting dispersion is then slowly stirred into three parts of the polyethylene emulsion, to form a homogeneous coating composition. This composition is then coated onto suitable backing, e.g. paper sheets, by any appropriate technique such as reverse-roll coating. The amount of coating thus applied is such as to provide a final dry coating of 10 to 40 pounds per 3000 square foot ream of the paper, and preferably about 20 pounds. The coating is dried and cured at 275 300 F. until the water and 2-amino-2- methylpropanol are driven off, and the polyethylene and the silicone are polymerized and set to their water-insoluble forms. The final coating thus contains about parts polyethylene, 18 parts stearic acid, 300 parts zinc oxide, and 8 parts silicone resin. The aforesaid drying and curing can be accomplished in 10-15 minutes at the aforesaid temperature, or alternatively can be accomplished by a shorter preliminary treatment (say 3-5 minutes) at such temperature followed by aging, at low humidity, in the roll. The thus-prepared photoconductive sheet gives excellent prints over a wide range of relative humidities, but is particularly suited for use at relative humidities of .as much as 80% and even higher.v

Example 2 The polyethylene emulsion of Example 1 containing 100 parts of the polyethylene resin is mixed with a dispersion of 450 parts zinc oxide, 450 parts water, and 5 parts silicone resin DC-772. Procedure is otherwise the same as in Example 1, and the resulting sheets likewise give good prints at similar humidities.

Examples 3-5 Polyethylene emulsions are prepared using polyethylene AC629', following the procedure of Example 1, but with the following proportion:

Exagiple Example Exa5rnple 4 Polyethylene AC-629... Oleie acid Stearie acid Ammonia Morpholine 7 Water Example 6 Forty parts of Epolene E12, the properties of which have already been given above, are melted, and 7 parts oleic acid added. The melt is cooled to 115 120 C. with continuous stirring. To this melt 7 parts morpholine are added, while maintaining the aforesaid temperature for one or two minutes after adding. The molten mix is then rapidly poured into 170 parts water at 95 C., with vigorous agitation. The resulting emulsion is then rapidly cooled to room temperature.

A zinc oxide dispersion is prepared by mixing 100 grams water, 5 parts water-soluble methyl silicone resin (DC772), and 140 parts Zinc oxide. After thorough mixing, the mixture is allowed to deaerate for an hour and then slowly added, with slow agitation, to the polyethylene emulsion.

Sheets prepared from the resulting composition, by the procedure described in Example 1, likewise give excellent prints even at humidities above 75 R.H.

Example 7 Example 6 showed the use of a low density water-emulsifiable polyethylene of low molecular weight. Examples 1-5 showed the use of such a polyethylene of intermediate molecular weight. This example shows the use of such a polyethylene of high molecular weight, as typified by Epolene E-15.

The properties of Epolene E-lS have already been given above. It is compounded with the other ingredients, in the following amounts:

Epolene E-lS 40 Oleic acid 7 Morpholine 7 Water 170 and in the manner described in Example 6. The resulting emulsion is combined with the zinc oxide dispersion in the manner and amounts shown in Example 6, and sheets coated with the product give equally good results at high humidities above 75% RH.

6 Example 8 This and the succeeding example show the use of high density water-emulsifiable polyethylenes in the present process. The polyethylene employed (Grex 39-2) has a molecular weight of about 7800, a density (25 C.) of

0.980.99, an acid number of 25, and a crystallinity of 92%.

Ingredients: Parts Grex 39-2 100 Oleic acid a 18 Morpholine 18 Water 250 The emulsifiers (oleic acid and morpholine) and the water are combined in a stirred vessel capable of withstanding 65 psi. The stirrer is started and the mixture heated to 95 C. with the vent open. The vent is then closed and the temperature raised to 150 C., and maintained at 150- 155 C. for 30 minutes, while stirring continues. The mixture is then cooled rapidly, while still stirring; the vent is opened when the temperature reaches 95 C. to reduce foaming. When the temperature has reached 70 C., the finished emulsion is removed.

A zinc oxide dispersion is prepared by mixing 350 parts water and 10 parts of water-soluble methyl silicone resin DC-772, and adding thereto 500 parts zinc oxide, with continued agitation. This mixture is allowed to stand for about an hour, and is then added slowly, with stirring, to the polyethylene emulsion.

The resulting composition is applied to a backing as in Example 1. The resulting sheets give equally satisfactory performance, even at relative humidities above 75%.

Example 9 The procedure of Example 8 is followed, using 18 parts stearic acid instead of 18 parts oleic acid, with substantially the same results.

The foregoing description and examples are intended' to be illustrative, and not in a limiting sense. Many variations will be obvious to those skilled in this art, within the scope of the claims that follow.

I claim:

1. A photoconductive sheet adapted to provide good prints under a wide range of humidity conditions including those in excess of 75 relative humidity, consisting essentially of a backing and a photoconductive coating on at least one surface thereof, said coating consisting essentially of parts of a polyethylene binder, about 15 to 30 parts of a water-insoluble fatty acid having 14 to 18 carbon atoms, 100 to 500 parts of a photoconductive substance in finely divided form, and a water-insoluble dispersant coating the particles of said substance, and present in an amount of about 0.5 to 5 percent thereof, said dispersant being a silicone formed from a watersoluble lower alkyl silicone resin upon drying and curing of said coating; parts being by weight; said polyethylene being, in the form applied as a coating, water-emulsifiable, at least 50% crystalline, and having an acid number of at least 10, and being selected from the group consisting of low-density polyethylenes having molecular weights between about 1200 and 3500 and softening points below C., and high-density polyethylenes having molecular weights between about 6000 and 9000 and softening points below C.

2. A photoconductive sheet in accordance with claim 1, wherein said photoconductive substance is zinc oxide.

3. A photoconductive sheet in accordance with claim 2, wherein said coating contains about 90 parts polyethylene binder, 18 parts fatty acid, 300 parts zinc oxide, and 8 parts silicone, parts being by weight.

4. A photoconductive coating material adapted to form the coating of a photoconductive sheet, consisting essentially of a homogeneous admixture of a water emulsion of water-emulsifiable polyethylene and a fatty acid having 14 to 18 carbon atoms, and a water dispersion of a photoconductive substance in finely-divided form and a water-soluble lower alkyl silicone resin which is convertible by heat to water-insoluble form, said fatty acid being present in an amount equal to about 15 to 30% of said polyethylene, by weight, and said silicone resin being present in an amount equal to about 0.5 to percent of said photoconductive substance by weight, the weight ratio of said polyethylene to said photoconductive substance being between 1:1 and 1:5; said polyethylene being at least 50% crystalline and having an acid number of at least 10, and being selected from the group consisting of low-density polyethylenes having molecular weights between about 1200 and 3500 and softening points below 115 C., and high-density polyethylenes having molecular weights between about 6000 and 9000 and softening points below 150 C.

5. A photoconductive coating material in accordance with claim 4, wherein said photoconductive substance is zinc oxide.

,6. A photoconductive coating material adapted to form the coating of a photoconductive sheet, consisting essentially of a homogeneous admixture of a water emulsion of water-emulsifiable polyethylene, a fatty acid having 14 to 18 carbon atoms, and a volatile organic base selected from the group consisting of amines and ammonia, and a water dispersion of zinc oxide and a watersoluble lower alkyl silicone resin which is convertible by heat to water-insoluble form, said fatty acid and said volatile organic base being present in an amount equal to about 25 to 50% of said polyethylene, by weight, and said silicone resin being present in an amount equal to about 0.5 to 5 percent of said zinc oxide by weight, the weight ratio of said polyethylene to said zinc oxide being between 1:1 and 1:5; said polyethylene being at least 50% crystalline and having an acid number of at least 10, and being selected from the group consisting of low- C., and high-density polyethylenes having molecular.

Weights between about 6000 and 9000 and softening pointsbelow 150 C.

7. Method of making a photoconductive sheet comprising the steps of preparing a water emulsion of a wateremulsifiable polyethylene, a fatty acid having 14 to 18 carbon atoms, and a volatile organic base selected from the group consisting of amines and ammonia, and a water dispersion of zinc oxide and a water-soluble lower alkyl silicone resin dispersant, mixing said emulsion and said dispersion uniformly together with sufficient water to adapt the resulting composition to a coating step, coating said mixture onto a backing sheet, and then drying and curing the resulting coating to drive off said water and said organic base and,to polymerize the polyethylene and convert the silicone resin to water-insoluble form and thereby provide a coated sheet adapted to make excellent prints by electrostatic printing technique at a wide range of relative humidities, more especially those in excess of relative humidity, the said composition containing, for each parts of said polyethylene, about 25 to 50 parts of said fatty acid and volatile organic base, 100 to 500 parts of said zinc oxide, and an amount of said silicone resin equal to about 0.5 to 5 percent of said zinc oxide, parts being by weight; said polyethylene being at least 50% crystalline and having an acid number of at least 10, and being selected from the group consisting of low-density polyethylenes having molecular weights between about 1200 and 3500 and softening points below C., and high-density polyethylenes having molecular weights between about 6000 and 9000 and softening points below C.

No references cited.

NORMAN G. TORCHIN. Primary Examiner. 

1. A PHOTOCONDUCTIVE SHEET ADAPTED TO PROVIDE GOOD PRINTS UNDER A WIDE RANGE OF HUMIDITY CONDITIONS INCLUDING THOSE IN EXCESS OF 75% RELATIVE HUMIDITY, CONSISTING ESSENTIALLY OF A BACKING AND A PHOTOCONDUCTIVE COATING ON AT LEAST ONE SURFACE THEREOF, SAID COATING CONSISTING ESSENTIALLY OF 100 PARTS OF A POLYETHYLENE BINDER, ABOUT 15 TO 30 PARTS OF A WATER-INSOLUBLE FATTY ACID HAVING 14 TO 18 CARBON ATOMS, 100 TO 500 PARTS OF A PHOTOCONDUCTIVE SUBSTANCE IN FINELY DIVIDED FORM, AND A WATER-INSOLUBLE DISPERSANT COATING THE PARTICLES OF SAID SUBSTANCE, AND PRESENT IN AN AMOUNT OF ABOUT 0.5 TO 5 PERCENT THEREOF, SAID DISPERSANT BEING A SILICONE FORMED FROM A WATER A WATERSOLUBLE LOWER ALKY SILICONE RESIN UPON DRYING AND CURING OF SAID COATING; PARTS BEING BY WEIGHT; SAID POLYETHYLENE BEING, IN THE FORM APPLIED AS A COATING, WATER-EMULSIFIABLE, AT LEAST 50% CRYSTALLINE, AND HAVING AN ACID NUMBER OF AT LEAST 10, AND BEING SELECTED FROM THE GROUP CONSISTING OF LOW-DENSITY POLYETHYLENES HAVING MOLECULAR WEIGHTS BETWEEN ABOUT 1200 AND 3500 AND SOFTENING POINTS BELOW 115*C., AND HIGH-DENSITY POLYETHYLENES HAVING MOLECULAR WEIGHTS BETWEEN ABOUT 6000 AND 9000 AND SOFTENING POINTS BELOW 150*C. 